There was so much written about the sharing of canteens and the refilling of them throughout the book that it was far too obvious IMO. Of every subplot in the book this one was the most transparent. Poison immediately entered my mind when the canteens were taken for filling.

I have to agree. I didn't really mind it at all, but the foreshadowing was a tad heavy-handed. Then again, I'm rarely surprised by things like this in books--unless it's utterly unforeshadowed and comes completely out of left field, that is. And I consider that a bit of a foul.